1. Technical Field of the Invention
This invention relates in general to a mobile communications system which has pagers and base stations providing two-way communication of alphanumeric messages between the pagers and the base stations. More particularly, these messages are communicated via the Global System for Mobile Communications (GSM/GPRS) network which has been set up on a world wide basis.
2. Description of the Prior art
Simple paging systems are comprised of small analogue radio receivers (pagers) and one or more transmitters (base stations). The base station in these systems transmits a series of tones which represent an address code and the pagers in the system evaluate the tones. At least one pager in the network detects the tone sequence as its own code and alarms the user of the pager with a beep or vibration that the user is being called.
Paging systems have progressed over the years in both frequency and extent of usage as well as complexity. Some pagers now have subminiature radio receivers, which are capable of reproducing a voice message which has been broadcast over the network by a transmitter. Only those pagers whose address codes have been transmitted over the network will open their audio paths to receive the transmitted voice message.
Some of the more recent pagers are equipped with a numeric or alphanumeric display. These pagers are capable of displaying a message sent by transmitters using a digital code which contains the address of a specific pager, or group of pagers, and the text of the message being sent. These paging systems may use a protocol such as the Post Office Code Standardization Advisory Group (POCSAG) or Golay Sequential Code (GSC). These protocols utilize well known error detection and error correction techniques and are, therefore, tolerant to bit errors occurring during transmission, provided the errors are not too numerous in any one word.
All of the paging systems described above are strictly one-way. There is no confirmation to the sender that a message has actually reached the pager, nor does the user have any means of sending an answer via his pager.
To remedy this limitation, pagers have been developed which allow two-way communication and which have individual transmission capability. When a pager of this type is called by a base station, the pager transmits its ID code to the base station which is an affirmative response that the pager has received the call.
A conventional pager which permits two-way data communication requires more electric power than can be provided by the battery of a conventional pager. In order to solve this problem, a dedicated network of repeaters are used to send the pager""s ID and message to the sending base station. A large number of repeaters are necessary for a two-way paging system to cover a large geographical area (e.g., a state or an entire country). This is a very expensive outlay needed to obtain a two-way data communication channel. In addition, a multi-repeater setup requires many different radio frequencies which are becoming more difficult to obtain from the FCC. Additional gateways are required if the communication system also accesses fax machines, the Internet or other alphanumeric devices. The dedicated two-way paging systems are ideal for a single-repeater configuration which covers a limited area (e.g., a single building or hospital).
Thus, what is needed is a two-way paging system that can be used throughout an entire country or internationally which does not require a large number of expensive repeaters. In addition, there is an increasing need to incorporate gateways to other communication networks within the paging system.
The Global System for Mobile Communications (GSM/GPRS) standard has been widely adopted in Europe and is now under consideration in several other countries outside of Europe. One of the primary goals of the GSM/GPRS standard is to enable users to move across national boundaries while still being able to communicate. Each country independently operates its own public land mobile network (PLMN) and the coverage of the PLMN is commercially confined to the borders of the country in which it is implemented. Radio coverage may overlap at national boundaries and each country may have several competing PLMNs.
When a GSM/GPRS customer subscribes to the GSM/GPRS system, the subscriber normally subscribes to a single PLMN, which is commonly referred to as the home-PLMN. The subscriber""s terminal, which is commonly referred to as the mobile station (MS), is typically thought of as having two principle components, namely, a first component relating to hardware and software for the radio interface and a second component relating to the subscriber identification information, which corresponds to the subscriber identity module (SIM). The SIM component can be removed from the subscriber""s mobile station and is typically the size of a credit card. The SIM is assigned a unique identity within the GSM/GPRS system, which is commonly referred to as the international mobile subscriber identity (IMSI). The IMSI is an identifier which is internal to the GSM/GPRS system and which is utilized by the GSM/GPRS system to identify the mobile station. A second identifier, commonly referred to as the temporary mobile station identity (TMSI), is assigned to each mobile station when it enters a new registration area and is valid only within that particular registration area. These identifiers should not be confused with the telephone number assigned to the mobile station. The TMSI and IMSI identifiers are utilized by the GSM/GPRS system to track the mobile station within a registration area and as it moves from one registration area into another registration area of the GSM/GPRS network.
Currently, a stand-alone, two-way paging system which utilizes the GSM/GPRS network does not exist. Although the GSM/GPRS standard does provide a short message service (SMS), which is suitable for alphanumeric paging, SMS is not widely utilized at present. The SMS provides for communication of alphanumeric messages at a bit rate which is much lower than the bit rate provided for cellular telephone communication over GSM/GPRS. However, in order for subscribers to have access to any of the GSM/GPRS services, including SMS, telephone companies require subscribers to pay cellular telephone rates. Although some companies provide cellular telephones with paging-type circuitry which can be used to transmit alphanumeric messages over the GSM/GPRS network, since GSM/GPRS subscribers are required to pay cellular telephone rates, GSM/GPRS subscribers typically communicate by voice over their cellular telephones rather than by sending alphanumeric messages over the lower bit rate SMS. Therefore, there has been no motivation or desire to provide stand-alone pagers for the GSM/GPRS network. All of these factors have resulted in a large amount of under-utilized SMS bandwidth.
In areas in which the number of available frequency bands is very limited, the typical pagers described above and the systems in which they operate are unsuitable due to the large number of frequency bands needed for their operation. For example, in small countries, such as Switzerland, these typical paging systems are unsuitable because the number of frequency bands required for their use is not available. It would be desirable to provide a two-way paging system which utilizes the currently under-utilized SMS bandwidth and which does not require the subscriber to have a cellular telephone. One of the advantages of such a system is that it would allow government emergency response agencies, such as the fire department, to have two-way communication over relatively simple paging devices and thus would eliminate the need to purchase cellular telephones for this purpose, which generally are much more expensive than pagers.
Accordingly, a need exists for a stand-alone, two-way pager which is capable of providing two-way communication over the GSM/GPRS network.
The present invention provides a communication system for transmitting messages from one or more dispatcher stations to and from one or more stand-alone, two-way pagers via a relatively low bit rate radio frequency communication channel, namely, the short message service (SMS) of the Global System for Mobile Communications (GSM/GPRS) cellular telephone network.
The paging system of the present invention utilizes a communications protocol which allows a dispatcher station and a pager of the paging system to communicate over the GSM/GPRS network in a predetermined manner. When the dispatcher station sends a message to a pager of the present invention, the message may contain steering codes which inform the pager of the manner in which the message is to be treated. For example, if the message is time-critical, the pager is notified by the steering code that a response to the message must be transmitted within a predetermined time interval. If the pager does not transmit a response within the predetermined time interval, the pager is not allowed to respond to the message.
The steering codes may also indicate whether or not the user of the pager must respond with a preprogrammed answer. If the steering code indicates that the user must respond with a preprogrammed answer which has been previously stored in the memory of the pager, the only response the user is allowed to transmit one of the preprogrammed answers. Otherwise, the user may transmit a different type of response, such as, for example, a message which the user types in using the alphanumeric keypad of the pager.
In accordance with a first embodiment of the present invention, the pager comprises only a few keys which allow the user to select from a limited number of preprogrammed answers to be sent to the dispatcher station in response to a message received by the pager. In accordance with a second embodiment of the present invention, the pager comprises a full alphanumeric keypad which allows the user to send messages which the user types in using the full alphanumeric keypad. The pager of the second embodiment may also be capable of sending preprogrammed answers.
Other features and advantages of the present invention will become apparent from the following description, drawings and claims.